Untying

As one of the 39 Melachot, untying
refers to the untying of any permanent
knot.
If a knot is not made to be permanent, it may
be untied. This is true even if it is
a permanent type knot. Thus, for example,
if one's shoes accidentally become knotted, they
may be untied.

Most of this information is taken
from the book "SABBATH Day of Eternity"
by Rabbi Aryeh Kaplan, published by NCSY.
In the book itself, there are notes that refer the
reader to detailed footnotes.
I am lucky to have found such a clear, concise manual
with these brief overviews of laws so vast and complex
there are Rabbis that specialize in them.